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Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
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Conjugated dienes have lower heats of hydrogenation than cumulated and isolated dienes, making them more stable. The enhanced stabilization of conjugated systems can be understood from their π molecular orbitals.
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Neutral hydrocarbons like cyclopentadiene with an odd number of carbon atoms and one intervening CH2 group in the ring are not aromatic. Cyclopentadiene with 4 π electrons does not satisfy the 4n + 2 π electron rule. Additionally, the intervening CH2 group is sp3 hybridized and lacks a vacant p orbital, thereby interrupting the overlap of p orbitals in a continuous manner and preventing the delocalization of π electrons throughout the ring.
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Published on: September 26, 2016

A π-stacked phenylacetylene dimer.

Surajit Maity1, G Naresh Patwari, Robert Sedlak

  • 1Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, India.

Physical Chemistry Chemical Physics : PCCP
|August 23, 2011
PubMed
Summary

Researchers elucidated the phenylacetylene-dimer structure using IR-UV spectroscopy and ab initio calculations. The study identified an anti-parallel π-stacked structure as the most stable configuration for the dimer.

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Area of Science:

  • Chemical Physics
  • Molecular Spectroscopy
  • Computational Chemistry

Background:

  • Understanding molecular dimers is crucial for predicting the behavior of larger systems.
  • Phenylacetylene is a fundamental aromatic alkyne with potential applications in materials science.

Purpose of the Study:

  • To determine the precise three-dimensional structure of the phenylacetylene-dimer.
  • To investigate the intermolecular interactions governing dimer formation.
  • To provide experimental and computational validation of the dimer's structure.

Main Methods:

  • Infrared-Ultraviolet (IR-UV) double resonance spectroscopy was employed to probe vibrational modes.
  • High-level ab initio calculations, specifically at the Coupled Cluster Singles Doubles with Perturbative Triples/Complete Basis Set (CCSD(T)/CBS) level, were performed.
  • Density Functional Theory Symmetry Adapted Perturbation Theory (DFT-SAPT) was used for energy decomposition analysis.

Main Results:

  • IR spectra indicated that both phenylacetylene units within the dimer experience similar environments, akin to the monomer.
  • Calculations favored an anti-parallel π-stacked structure, supported by stabilization and free energy analyses.
  • DFT-SAPT analysis revealed that this anti-parallel arrangement optimizes both electrostatic and dispersion energy contributions.
  • Experimental IR spectra strongly correlated with the predicted anti-parallel π-stacked conformation.

Conclusions:

  • The phenylacetylene-dimer predominantly adopts an anti-parallel π-stacked structure.
  • This structure is stabilized by favorable electrostatic and dispersion interactions.
  • The combination of IR-UV spectroscopy and advanced computational methods reliably elucidates dimer structures.